1. Field of the Invention
The field of the invention is photosensitive switches and in particular photosensitive switches for reading registration marks and having an alarm output to indicate a loss of detection of those marks.
2. Background Art
Photosensitive switches are used in the processing of printed materials. Registration marks, typically on the border of a continuous web of printed paper, are detected by the photosensitive switch to produce a control signal. The control signal is used to activate and synchronize packaging machines that receive the web and then cut, seal, crimp, or fold the web material at the locations indicated by the registration marks.
Reliable detection of the registration marks requires that three conditions be met: the registration marks must have good contrast with respect the background web material, the web and registration marks must be accurately aligned with the photosensitive switch, and the optical path between the switch and the web must be clear of obstructions, in particular, the lens of the photosensitive switch must be clear of dust and the like. These conditions may change adversely during operation and therefore it is essential that the photosensitive switch provide a indication if the detection of the registration marks is lost so that appropriate corrections may be made.
One method of determining whether the detection signal has been lost is through the use of a "watch-dog" timer. A watchdog timer measures the time interval between the detection of each registration mark and produces an alarm signal if that interval exceeds a predetermined time limit.
One drawback to this use of a watch-dog timer is that the appropriate time limit depends on the web speed and on the spacing of the registration marks. The time limit must be changed if either the web speed or the mark spacing is changed. If several photosensitive switches are used on a single piece of equipment this adjustment of the time limit may be cumbersome. With any fixed time limit, the stopping of the equipment momentarily, or the operation of the equipment in step mode, will cause the alarms to be activated.
A second drawback to the use of a watchdog timer is that the optimum time limit may vary widely as a function of the product being produced. For this reason, these time limits may not be preset during manufacture of the photosensitive switch, but must be adjusted in the field. This requires the provision of readily accessible controls on each photosensitive switch for easily and accurately changing the predetermined time limit. Such controls increase the cost of the photosensitive switch.
A further drawback to an alarm using a watch dog timer is that it provides a signal only after the detection of registration marks is lost completely. Ideally, an alarm should indicate a problem condition prior to the complete loss of detection so that preventative measures can be taken.